Swindler: +2 Coins, Each other player trashes the top card of his deck and gains a card with the same cost that you choose.
Say my opponent has played a Black Market and now the top card of their deck is a 4-cost card they purchased with it. There are no cards in the kingdom set that cost 4, and I play my Swindler. My opponent trashes the 4-cost card from the top of his deck, but what, if anything, does he gain since there are no other 4-cost cards in the kingdom set?